This application is related to Japanese Patent Application No. 2000-124311, filed on Apr. 25, 2000 whose priority is claimed under 35 USC xc2xa7119, the disclosures of which are incorporated by reference in their entirety.
1. Field of the Invention
The present invention relates to a method for producing a semiconductor device, and in further detail, it relates to a method for producing a semiconductor device comprising a gate electrode having a metallic silicide layer formed on the surface thereof.
2. Prior Art
In producing MOS transistors, gate electrodes and source/drain regions with low resistance have been implemented heretofore by employing a method comprising forming silicide films on the surface thereof (see, for instance, an unexamined published Japanese patent application Hei6 (1994)-132243).
In general, silicide is formed on the surface of the gate electrode and source/drain regions in accordance with the following procedure.
Referring first to FIG. 7(a), a silicon oxide film and a polysilicon film are formed on an N well 33 formed on a silicon substrate 31 having an device isolation region 32 provided thereon, and after patterning the silicon oxide film and the polysilicon film as desired, a gate oxide film 34 and a gate electrode 35 are formed. A side wall spacer 36 is formed on the side wall of the gate electrode 35 thereafter.
Then, as shown in FIG. 7(b), the surface of the silicon substrate 31 is subjected to ion implantation of boron difluoride 37 by using the gate electrode 35 and the side wall spacer 36 as masks. Then, the source/drain regions 38 are formed by applying thermal treatment thereto. By the ion implantation, boron difluoride 37 is implanted to the gate electrode 35, at the same time, contaminants such as tungsten, etc., derived from members constituting the ion implantation apparatus, are incorporated into the gate electrode 35 and the source/drain regions 38.
Referring to FIG. 7(c), the contaminants 40 remaining on the upper surface of the gate electrode 35 and on the source/drain regions 38 that are not covered by the side wall spacer 36 are volatilized by heating with a lamp in an inert gas.
Then, the silicon of the gate electrode 35 and the source/drain regions 38 is exposed by removing the naturally oxidized film 39 on the surface of the gate electrode 35 and the source/drain regions 38 by using argon ion sputtering.
Subsequently, as shown in FIG. 7(d), a titanium film 41 is vapor deposited on the entire surface of the silicon substrate 31. Then referring to FIG. 7(e), titanium is reacted with silicon by heating, and after removing the titanium film 41 remaining unreacted, a titanium silicide film 42 is formed on the surface of the gate electrode 35 and the source/drain regions 38.
In the case where a MOS transistor is produced by forming the titanium silicide film 42 in accordance with the method above, however, the MOS transistor sometimes suffers an extremely low reliability ascribed to the deterioration of the gate oxide film 34.
This is attributed to the fact that the contaminants such as tungsten, etc., that are incorporated during the ion implantation in forming the gate electrode and the source/drain regions induce an excessive reaction inside the gate electrode and the source/drain regions by the heat treatment for forming the silicide film.
More specifically, ion implantation is generally performed by using an ion implantation apparatus comprising an ion source comprised an arc chamber and a filament which are made of tungsten. Tungsten from the ion source generates divalent tungsten fluoride ions during the implantation of boron difluoride ions. These divalent ions undergo change in charge after passing through the draw out electrode to form trivalent ions before entering the magnet of mass analyzer. Thus are formed tungsten monofluoride ions (WF+++, having a mass number of 44.2 to 45.6), tungsten difluoride ions (WF2+++, having a mass number of 48.4 to 49.9), etc. Hence, it is believed that, because these trivalent tungsten fluoride ions have a mass number near to 49, i.e., the mass number of boron difluoride, they cannot be completely removed at the slit of the mass analyzer, and are thereby implanted into the silicon substrate (see, for example, xe2x80x9cContamination in ULSI Productionxe2x80x9d (Tsuneo Ajioka et al., July 1999, pp. 304-311)).